1. Field of the Invention
The present invention relates to a novel cruise control system for motorcars, in particular to a cruise control system which can be advantageously used to follow a preceding car on highways.
The cruise control system according to the present invention is so simple that is easily operable by any driver without any feeling of disorder or inconvenience and improves fuel economy.
2. Description of the Related Art
A variety cruise control systems are known and are equipped in actual motorcars. All known cruise control systems, however, are designed for a leading car which cruises at the head of a group of cars going in the same direction at a constant velocity at driver's pace, but are not designed for a car which follows other cars. In other words, conventional cruise control systems are not convenient for a "following driving mode".
In conventional cruise control systems, when a driver of a following car intends to follow a preceding car, firstly he must drive his car so as to keep a constant distance from the preceding car and then, when the driver judges that the velocity of his car is equal to that of the preceding car, he pushes a setting button of the cruise control system to start the cruise control system. However, he is usually soon requested to readjust or correct the cruise control system, because the distance to the preceding car estimated might not be correct and will drift or go out of a target distance. In this case, driver must simultaneously perform two corrections or readjustments of (1) the distance to the preceding car and (2) fine correction of the car velocity.
The correction or readjustment of known cruise control systems, however, is very complicated or sophisticated and is not easy. In fact, when the distance to the preceding car becomes shorter, firstly the driver must disengage the cruise control system to put the accelerator pedal back, so that the car slows down and then, when he judges that a proper distance to the preceding car is recovered, he pushes a restoring button so as to return to the velocity value previously set. In this case, however, the cruise control system commands the accelerator to open to its full potential because the car's velocity has slowed down below the set velocity beyond the ordinary control range, resulting in that automatic transmission is kicked down to carry out a sudden acceleration. After this acceleration is complete and the car returns to a stable velocity, the driver is requested to cancel the cruise control system once then to decelerate a little before he resets the cruise control system again. The conventional cruise control system requires such complicated controlling procedure of drivers. Still further, the above-mentioned small deceleration must be effected by driver's own perception, since the speed meter does not have such a high resolution that can measure such a small decrease in velocity. This necessitates readjustment often. If a driver who hates such complicated procedure cancels the cruise control system abruptly to increase the distance to the preceding car, the set value input previously is lost or disappears and hence the driver can't utilize it, resulting in that precision in velocity is returned to the starting point.
In an opposite case wherein the distance to the preceding car becomes longer than a target value, the accelerator pedal is depressed to recover the target distance to the preceding car. Then, when the accelerator pedal is released, deceleration starts because the cruise control system commands the accelerator to stop accelerating in this case. So, after the cruise control system returns to its balanced condition, the driver pushes an incremental button to gradually increase the target velocity value.
The above-mentioned complicated procedures may be possible theoretically but are not realistic, so conventional cruise control systems are not convenient for drivers and are not used in actual driving. Impracticability of the conventional cruise control system come mainly from following two reasons.
Firstly, frequent correction of adjustment is not expected in the cruise control system. In other words, in "following driving mode", a following car controlled by the cruise control system must run at least more than several ten times longer time duration than manual driving in which a driver can adjust continuously the car velocity. This means that a driver needs to input a velocity value to the cruise control system with such extremely high precision in velocity that is more than one several tenth more sensitive than manual driving. Human beings do not possess such high resolution in velocity. Such fine control can be realized only by a computer having a special program to follow a preceding car.
Secondly, in the conventional cruise control system which is based on the velocity control theory, an operational factor which a driver can input to the system is a single velocity value. In other words, the conventional cruise control system is designed mainly for leading cars but not for following car or tailing cars. Therefore, the conventional cruise control system can not be used conveniently in the "following driving mode". It is not easy to realize such a control system that can control or adjust both the distance to a car in front and the velocity simultaneously, because these two factors are related to each other and must be operated sequentially and automatically by a single input signal.
The distance to the preceding car can be measured directly by using a radar or other sensors. Automatic cruise control systems using such measuring means are expected theoretically but it is not realistic at the present level of technology for many reasons.
The conventional cruise control system has other demerits also. Firstly, in the conventional cruise control system based on the velocity control, deviation or drift in velocity from a set value caused by change in resistance to which the car is subjected can be recovered automatically but the time elapsed can never be recovered. In other words, accumulation of time duration for compensating such time elapsed caused by errors in the velocity result in a delay in the arrival time at the destination. Secondly, in a car equipped with the conventional cruise control system, it is often observed that a driver operates an acceleration pedal much more frequently than in manual driving, because the above-mentioned inconvenience of the known system. This means that the car wastes fuel and, in the case of a car equipped with a torque convertor, unnecessary "kick-down" occurs on up-hill roads where the top gear can be used in the case of manual driving mode, resulting in bad ride. Finally, frequent changes of the gear ratio up and down are often observed in the neighborhood of a gear shifting point, when a car equipped with a torque convertor runs on up-hill roads. Such shift can be prevented by modifying the velocity in the case of manual driving. However, since the conventional cruise control system is not convenient in such situation, the driver gives it up and resets the system.
In conclusion, the conventional cruise control system has been considered to be one of non-useful decoration for high-class motorcars but has been forgotten by popular cars or business cars in Japan.
Therefore, an object of the present invention is to provide a novel cruise control system for motorcars which can be used advantageously in following driving mode and of course in leading driving mode also, which permits to follow to a preceding car easily, stably and safely and which can reduce driver's load and increase economy of fuel.